1. Field of the Invention
This invention relates to apparatus and methods for encoding and decoding information in analog signals, such as audio, video and data signals, either transmitted by radio wave transmission or wired transmission, or stored in a recording medium such as optical or magnetic disks, magnetic tape, or solid state memory.
2. Background and Description of Related Art
An area of particular interest to certain embodiments of the present invention relates to the market for musical recordings. Currently, a large number of people listen to musical recordings on radio or television. They often hear a recording which they like enough to purchase, but don't know the name of the song, the artist performing it, or the record, tape, or CD album of which it is part. As a result, the number of recordings which people purchase is less than it otherwise would be if there was a simple way for people to identify which of the recordings that they hear on the radio or TV they wish to purchase.
Another area of interest to certain embodiments of the invention is copy control. There is currently a large market for audio software products, such as musical recordings. One of the problems in this market is the ease of copying such products without paying those who produce them. This problem is becoming particularly troublesome with the advent of recording techniques, such as digital audio tape (DAT), which make it possible for copies to be of very high quality. Thus it would be desirable to develop a scheme which would prevent the unauthorized copying of audio recordings, including the unauthorized copying of audio works broadcast over the airwaves. It is also desirable for copyright enforcement to be able to insert into program material such as audio or video signals digital copyright information identifying the copyright holder, which information may be detected by appropriate apparatus to identify the copyright owner of the program, while remaining imperceptible to the listener or viewer.
Various prior art methods of encoding additional information onto a source signal are known. For example, it is known to pulse-width modulate a signal to provide a common or encoded signal carrying at least two information portions or other useful portions. In U.S. Pat. No. 4,497,060 to Yang (1985) binary data is transmitted as a signal having two differing pulse-widths to represent logical "0" and "1" (e.g., the pulse-width durations for a "1" are twice the duration for a "0"). This correspondence also enables the determination of a clocking signal.
U.S. Pat. No. 4,937,807 to Weitz et al. (1990) discloses a method and apparatus for encoding signals for producing sound transmissions with digital information to enable addressing the stored representation of such signals. Specifically, the apparatus in Weitz et al. converts an analog signal for producing such sound transmissions to clocked digital signals comprising for each channel an audio data stream, a step-size stream and an emphasis stream.
With respect to systems in which audio signals produce audio transmissions, U.S. Pat. Nos. 4,876,617 to Best et al. (1989) and 5,113,437 to Best et al. (1992) disclose encoders for forming relatively thin and shallow (e.g., 150 Hz wide and 50 dB deep) notches in mid-range frequencies of an audio signal. The earlier of these patents discloses paired notch filters centered about the 2883 Hz and 3417 Hz frequencies; the later patent discloses notch filters but with randomly varying frequency pairs to discourage erasure or inhibit filtering of the information added to the notches. The encoders then add digital information in the form of signals in the lower frequency indicating a "0" and in the higher frequency a "1". In the later Best et al. patent an encoder samples the audio signal, delays the signal while calculating the signal level, and determines during the delay whether or not to add the data signal and, if so, at what signal level. The later Best et al. patent also notes that the "pseudo-random manner" in moving the notches makes the data signals more difficult to detect audibly.
Other prior art techniques employ the psychoacoustic model of the human perception characteristic to insert modulated or unmodulated tones into a host signal such that they will be masked by existing signal components and thus not perceived. See, e.g. Preuss et al., U.S. Pat. No. 5,319,735, and Jensen et al., U.S. Pat. No. 5,450,490. Such techniques are very expensive and complicated to implement, while suffering from a lack of robustness in the face of signal distortions imposed by perception-based compression schemes designed to eliminate masked signal components.
The prior art fails to provide a method and an apparatus for encoding and decoding auxiliary analog or digital information signals onto analog audio or video frequency signals for producing humanly perceived transmissions (i.e., sounds or images) such that the audio or video frequency signals produce substantially identical humanly perceived transmission prior to as well as after encoding with the auxiliary signals. The prior art also fails to provide relatively simple apparatus and methods for encoding and decoding audio or video frequency signals for producing humanly perceived audio transmissions with signals defining digital information. The prior art also fails to disclose a method and apparatus for limiting unauthorized copying of audio or video frequency signals for producing humanly perceived audio transmissions.